1. Field of the Invention
The present invention relates to a zoom lens, more particularly to a spring-biasing structure in a drive mechanism for driving optical elements of the zoom lens such as lens groups.
2. Description of the Related Art
In drive mechanisms of zoom lenses for driving optical elements such as lens groups, it is generally the case that a plurality of movable frames which are moved in an optical axis direction relative to one another in different moving manners are biased mutually by biasing springs to remove backlash in the plurality of movable frames. Specifically, there is a high possibility of an external force being exerted on one or more external barrels of the zoom lens, and accordingly, external barrels of the zoom lens need to be biased as appropriate by strong biasing forces to be capable of returning to respective normal positions of the external lens barrels when undergoing displacement by an external force. In telescopic zoom lenses having a plurality of external barrels which are concentrically arranged, it is conventional for the frontmost external barrel (usually the radially-innermost external barrel) to hold a lens group having the largest diameter among all the lens groups of a photographing optical system, so that the frontmost external barrel tends to be heavier in weight than the other movable barrels or frames. Therefore, such an external barrel is often required to be biased by a strong biasing spring force in order to be held with stability.
In the case where three movable members (a frontmost movable member, an intermediate movable member and a rearmost movable member) which are aligned along an optical axis are mutually spring-biased, it is conventional for one end of each two biasing springs to be fixed to the frontmost movable member and the rearmost movable member, respectively, and the other ends of the two biasing springs are fixed to the intermediate movable member.
However, the biasing spring forces of the biasing springs vary due to the difference in amount of movement among the movable frames including the aforementioned one or more external barrels, which makes it difficult to apply a stable biasing spring force to each movable frame. For instance, since the minimum biasing force among variations in the biasing spring force exerted on the movable frames needs to be determined as a reference spring load (minimum return force), the spring load grows excessively upon the spring biasing force becoming maximum, and may become a great burden on operations of the zoom lens. As a result, in the case of a motor-driven zoom lens, a zoom motor thereof needs to be a large type producing a strong torque, which makes it difficult to achieve miniaturization and weight reduction of the zoom lens.
In the above described structure in which the two biasing springs are arranged on the front side and the rear side of the intermediate movable member, respectively, each biasing spring cannot have a greater length than the distance between associated two movable members adjacent to the biasing spring in the optical axis direction, and therefore needs to be a biasing spring having a relatively large spring constant. However, using such a biasing spring makes it difficult to assemble the zoom lens. Additionally, in this structure it is difficult to achieve the balance between the two biasing springs.